Predicting bad pages in NAND flash to improve read time: A dynamic programming approach

One of the main attractions for the widespread of flash memories in data storage is their fast read speed. Read speed can potentially be jeopardized when there are a large number of read errors as a result of flash cell wear-and-tear, leakage, and many other factors. Powerful (error-correction coding) ECC techniques are generally employed, but the number of errors that these schemes can correct is limited; therefore, often other sophisticated error correction mechanisms such as RAID need to be invoked, as well, which introduce significant latency. Flash pages that are not ECC correctable are identified as bad pages. By proactively reading pages, decoding the data and comparing the bit-error with a pre-fixed bit-error threshold value one can potentially predict bad pages and remove their content before they become problematic. This paper proposes finding such bit-error thresholds by formulating the proactive read process as a Markov Decision Process (MDP). Based on Dynamic Programming (DP) an algorithm can then be obtained that selects a maximum bit-error value (threshold) above which pages should be removed. Such threshold values can simply be stored as a look-up table in the controller ahead of time.